Japanese Patent Publication No. 2000-313322 discloses a related vehicular brake control device that aims to achieve a good brake feeling when a manual brake is operated during automatic braking. The brake control device has a relief valve for releasing brake fluid, and is thus able to release brake fluid pressure applied to a wheel cylinder by the manual brake. When a brake pedal is operated during automatic braking, the opening of the relief valve is controlled in accordance with a release time. This release time is determined based on a vehicle body deceleration, and corrected using a depression speed of the brake pedal. Thus, a pedal stroke that corresponds with the manual braking state can be obtained.
However, according to the disclosure of Japanese Patent Publication No. 2000-313322, when the brake fluid pressure applied to the wheel cylinder is released by the operation of the manual brake, the deceleration speed generated by the automatic brake is temporarily reduced, and deceleration speed inconsistency, namely, a fall in the deceleration speed, may result.
For example, if there is a risk of a frontal collision with an obstacle in front of the vehicle, the likelihood that a collision will occur (hereinafter referred to as the “frontal collision likelihood level”) during operation of the automatic brake increases, and thus the driver may operate the manual brake. At this time, the automatic brake is stopped, and brake fluid pressure is applied to the wheel cylinder based on a master cylinder pressure generated by the operation of the manual brake. Thus, when an operation amount of the manual brake by the driver is small, even if the corresponding brake fluid pressure is applied to the wheel cylinder, the brake fluid pressure applied is smaller than that previously generated by the automatic brake. Therefore, even if the driver actually operates the manual brake, the deceleration speed is reduced.
As a result of this reduction, the driver feels a sense of unease, i.e., feels that the braking force is weakened, and is not able to obtain a brake feeling that meets with the driver's expectations.
Moreover, when the above brake control device performs the brake assist control, if the frontal collision likelihood level increases during operation of the automatic brake, and the driver strongly depresses the brake pedal, the automatic brake is stopped, and the brake assist control is started. At this time, the amount of the pressure applied to the wheel cylinder is boosted in accordance with the driver's operation of the brake pedal, and high brake pressure is suddenly applied to the wheel cylinder.
In this case, the vehicle body deceleration is increased, and the driver may feel a sense of unease due to the sudden braking force. In particular, if the brake control device is configured such that (i) the frontal collision likelihood level is determined based on CCD camera detection or the like, and (ii) a control amount generated by the brake assist control is adjusted by a pre-crash control in accordance with the frontal collision likelihood level, the control amount is adjusted to a higher amount when the collision likelihood level is high. Thus, when the driver feels that there is a risk of collision, and depresses the brake pedal strongly, the pressure applied to the wheel cylinder may be increased rapidly.